Forming contacts on semiconductor substrates, radiation detectors and imaging devices

ABSTRACT

A method, suitable for forming metal contacts on a semiconductor substrate at positions for defining radiation detector cells, includes the steps of forming one or more layers of material on a surface of the substrate with openings to the substrate surface at the contact positions; forming a layer of metal over the layer(s) of material and the openings; and removing metal overlying the layer(s) of material to separate individual contacts. Optionally, a passivation layer to be left between individual contacts on the substrate surface may be applied. Etchants used for removing unwanted gold (or other contact matter) are preferably prevented from coming into contact with the surface of the substrate, thereby avoiding degradation of the resistive properties of the substrate.

This is a divisional of application Ser. No. 08/755,826, filed on Nov.26, 1996, now U.S. Pat. No. 6,046,068.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to methods for manufacturing radiation detectorsand radiation imaging devices, radiation detectors and imaging devicesmanufactured by such methods, and the use of such radiation detectorsand imaging devices.

2. Description of the Prior Art

A typical method of manufacturing a radiation detector for an imagingdevice comprises applying a layer of a metal such as aluminum to both ofthe main surfaces of a planar semiconductor substrate, applying a layerof photoresistive material to cover the semiconductor material, exposingthe photoresistive material on the surface of the planar substrate withan appropriate mask pattern, removing the photoresistive material toexpose a pattern of the metal to be removed, etching away the metal tobe removed, and then removing the remaining photoresistive material toleave a pattern of contacts on one surface of the substrate and ametallized layer on the other surface of the substrate. The contacts onthe first surface of the substrate then define an arrangement ofradiation detector cells.

For optical wavelengths and charged radiation (beta-rays), silicon hastypically been used for the semiconductor material for the substrate. Amethod of the type described above has been used to good effect withthis material.

In recent years, cadmium zinc telluride (CdZnTe) has increasingly beenproposed as a more suitable semiconductor material for use in X-ray,gamma-ray and, to a lesser extent, beta-ray radiation imaging. CdZnTe isgood at absorbing X-rays and gamma-rays, giving better than 90%efficiency for 100 keV X-rays and gamma-rays with a 2 mm thick detector.The leakage or dark current of these detectors can be controlled, andvalues on the order of 10 nA/cm² or less at 100 Volts bias areachievable.

A small number of companies worldwide currently produce these detectorscommercially in a variety of sizes and thicknesses. Usually one or bothsides of the planar detectors are contacted with a continuous metallayer such as gold (Au) or platinum (Pt). As mentioned above, suchdetector substrates then need to be processed to produce a detectorhaving a pattern of contacts (e.g., pixel pads) on one surface, with theopposite surface remaining uniformly metallized, in order that thedetector may be position sensitive (that is, in order that the detectoris able to produce a detector output indicating the position at whichradiation impacts the detector). A readout chip then can be “flip-chip”joined to the patterned side of the CdZnTe detector (e.g., by bumpbonding using balls of indium or conductive polymer material, gluingusing one-way conductive materials, or other conductive adhesive layertechniques) so that the position-dependent electrical signals whichresult from incidence and absorption in the detector cells of X-rays orgamma-rays can be processed. The readout chip could be of the pulsecounting type with very fast integration and processing time (typicallya few microseconds, or at most a few milliseconds). Alternatively, itmay be one of a type described in the Applicants's International patentapplication PCT EP95/02056 which provides for charge accumulation forindividual detector cells, the disclosure of which is expresslyincorporated herein by reference. With an imaging device as described inPCT EP95/02056, integration times can be several milliseconds, or tensor hundreds of milliseconds. As the signal integration orstandby/readout period increases it becomes more critical that the goldor platinum contacts on the CdZnTe surface are electrically separated toa high degree to avoid signals from neighboring contacts (pixel pads)leaking and causing the contrast resolution to degrade.

It has been found that traditional methods of forming contacts on adetector surface, particularly when CdZnTe is used as the semiconductormaterial, do not provide as high an electrical separation of thecontacts as is desired to make optimum use of the advantages which areto be derived from the imaging devices as described in the Internationalpatent application PCT EP95/02056.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a method forforming metal structures (e.g., metal contacts) on a semiconductorsubstrate at spaced positions (e.g., for defining radiation detectorcells) includes the steps of forming one or more layers of material on asurface of the substrate with openings to the substrate surface at thecontact positions; forming a layer of metal over the layer(s) ofmaterial and the openings; and removing metal overlying the layer(s) ofmaterial to separate individual contacts.

The present inventors have found that the surface resistivity of aCdZnTe semiconductor substrate is degraded when the substrate is exposedto metal etchants suitable for removing gold and/or platinum. As aresult, the electrical separation of the individual contacts whichresult from the conventional method of forming such contacts is not asgood as would be expected from the properties of that material beforetreatment. By using a method in accordance with the present invention,the surface of the semiconductor substrate between the contacts can beisolated from the metal etchants, thus preventing the damage which wouldresult if the metal etchants came into contact with the semiconductorsurface.

Thus, in this first embodiment, the step of forming one or more layersof material on a surface of the substrate with openings to the substratesurface at the contact positions may include the sub-steps of forming alayer of photoresistive material on the substrate surface; andselectively exposing the photoresistive material, and removing thephotoresistive material from areas corresponding to the contactpositions to expose the substrate surface.

Alternatively, this step may include the sub-steps of forming a layer ofpassivation material on the substrate surface; forming a layer ofphotoresistive material on the passivation layer; selectively exposingthe photoresistive material, and removing the photoresistive materialfrom areas corresponding to the contact positions to expose thepassivation material layer; and removing the passivation material fromthe exposed areas corresponding to the contact positions to expose thesubstrate surface. The use of an insulating layer of passivationmaterial means that after manufacture of the detector, the passivationmaterial remains between the contacts, protecting the semiconductorsurface from environmental damage in use and further enhancing theelectrical separation of the contacts. To protect the other main surfaceand the sides (edges) of the semiconductor substrate, photoresistivematerial can additionally be applied to all exposed surfaces prior tothe step of removing the passivation material from the exposed areascorresponding to the contact positions.

Again with reference to the above-referenced first embodiment, the stepof removing metal overlying the layers of material to separateindividual contacts may include the steps of forming a further layer ofphotoresistive material on at least the metal layer; selectivelyexposing the photoresistive material of the further layer, and removingthe photoresistive material of the further layer apart from areascorresponding generally to the openings; and removing metal not coveredby the photoresistive material of the further layer. In addition, anyremaining photoresistive material may be removed. In a variation on thisembodiment, the areas corresponding generally to the openings are largerthan the corresponding openings, so that after removal of the metal notcovered by the photoresistive material of the further layer, thecontacts cover the opening and also extend up and laterally beyond theopening. In this way the ingress of metal etchant around thephotoresistive material, whereby the metal etchant might reach thesemiconductor surface, can be avoided.

The present invention is particularly useful with substrates formed ofcadmium zinc telluride (CdZnTe). Nevertheless, persons skilled in theart will appreciate that methods according to embodiments of the presentinvention can readily be used with other substrate materials.

The metal layer for forming the contacts is preferably applied bysputtering, but other methods, including evaporation and electrolyticdeposition, may also be used. The metal layer itself preferablycomprises gold (Au), although other metals, such as platinum (Pt) orindium (In), may also be used. The passivation layer preferablycomprises aluminum nitride (AlN).

Again with reference to the above-referenced first embodiment, the stepof removing metal overlying the layers of material to separateindividual contacts may comprise removing unwanted metal by aphotoresist liftoff technique. More typically, however, removal ofunwanted metal may be accomplished using an appropriate metal etchant.

According to another embodiment of the present invention, each metalcontact can define a respective pixel cell of an array of pixel cells,or one of a plurality of strips arranged parallel to one another,depending on the application of the detector. Using a method accordingto the present invention, it is possible to configure metal contacts onthe order of 10 μm across with a spacing on the order of 5 μm.

Yet another embodiment of the present invention provides a method ofmanufacturing a radiation detector comprising a semiconductor substratewith a plurality of metal contacts for respective radiation detectorcells on a first surface thereof and layer of metallization on a surfaceof the substrate opposite to the first surface. The metal contacts maybe formed on the first surface using a method such as the firstembodiment described above. In such an implementation, the layer ofmetallization can be formed on the opposite surface of the substrateprior to applying a layer of material to the upper surface of thesubstrate.

Yet another embodiment of the present invention provides a method ofmanufacturing a radiation imaging device. According to this embodiment,a radiation detector may be manufactured using one of the methodsreferenced above. Individual contacts for respective detector cells maythen be individually connected to corresponding circuits on a readoutchip by, for example, a “flip-chip” technique.

According to another embodiment of the present invention, a radiationdetector may be formed with a semiconductor substrate having a pluralityof metal contacts for respective radiation detector cells on a firstsurface thereof and a layer of metallization on a surface of thesubstrate opposite to the first surface, wherein the overall width ofthe metal contacts is larger than the width of the contact adjacent thesubstrate.

The semiconductor substrate of the various embodiments referenced aboveis preferably made of cadmium zinc telluride (CdZnTe), although othermaterials, such as cadmium telluride (CdTe), may also be used. Inaddition, passivation material is preferably provided between individualcontacts. Aluminum nitride has been found to be particularly effectiveas a passivation material for CdZnTe, which is temperature sensitive,because it can be applied at low temperatures.

Another embodiment of the present invention provides a radiationdetector comprising a semiconductor substrate with a plurality of metalcontacts for respective radiation detector cells on a first surfacethereof and a layer of passivation material on the surface between themetal contacts, the passivation material comprising aluminum nitride.The metal contacts can define an array of pixel cells, or a plurality ofstrips arranged parallel to one another, depending on the field of useof the detector. Pixel contacts formed on a detector substrate arepreferably substantially circular and arranged in a plurality of rows,with alternate rows ideally being offset from adjacent rows. The metalcontacts may be on the order of 10 μm across with a spacing on the orderof 5 μm. In detectors in accordance with embodiments of the presentinvention, the resistivity between metal contacts should be in excess of1 GΩ/square, preferably in excess of 10 GΩ/square, more preferably inexcess of 100 GΩ/square, and even more preferably in excess of 1000GΩ/square (1 TΩ/square).

Yet another embodiment of the present invention provides a radiationimaging device comprising a radiation detector as defined above and areadout chip having circuits for accumulating charge from successiveradiation hits, with individual contacts for respective detector cellsbeing connected by a “flip-chip” technique to respective circuits foraccumulating charge. Such a radiation imaging device may be particularlyuseful for X-ray, gamma-ray and beta-ray imaging.

Another embodiment of the present invention provides a method ofmanufacturing, for example, detectors having a CdZnTe substrate with oneside uniformly metallized with gold and the other side patterned withgold structures in a manner that does not adversely affect the surfacecharacteristics of the CdZnTe substrate between the gold structures.Using such a method, it is possible to create gold structures on oneside of a CdZnTe detector and achieve inter-structure resistivity on theorder of GΩ/square (or tens or hundreds of GΩ/square).

The use of an electrically-insulating passivation layer between contactsfurther enables the area between metal contacts to be protected, thusgiving a detector according to an embodiment of the present inventionstable performance over time and avoiding effects such as oxidationwhich increase the surface leakage current and decrease theinter-contact resistivity. Aluminum nitride (AlN) passivation has beenfound to be particularly effective when applied between gold contacts toprotect the surface and enhance the electrical separation of the goldcontacts. The passivation layer of aluminum nitride can be implementedat relatively low temperatures, typically less than 100° C. By contrast,silicon oxide (SiO₂), which is typically used as a passivant for silicon(Si) semiconductors, needs temperatures in excess of 200° C. CdZnTewould be unusable after exposure to such temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1I illustrate in progressive fashion a method for forming metalcontacts on a semiconductor substrate, according to an embodiment of thepresent invention.

FIGS. 2A-2K illustrate in progressive fashion a method for forming metalcontacts on a semiconductor substrate with a passivation layer betweencontacts, according to an embodiment of the present invention.

FIGS. 3A-3L illustrate in progressive fashion a second method forforming metal contacts on a semiconductor substrate with a passivationlayer between contacts, according to an embodiment of the presentinvention.

FIG. 4 is a schematic plan view of a first contact configuration on adetector substrate.

FIG. 5 is a schematic plan view of a second contact configuration on adetector substrate.

FIG. 6 is a schematic plan view of a third contact configuration on adetector substrate.

FIG. 7 is a schematic plan view of a radiation imaging device includinga radiation detector and a readout chip.

DETAILED DESCRIPTION

FIGS. 1A-1I illustrate in progressive fashion a method for forming metalcontacts on a semiconductor substrate at positions for definingradiation detector cells, according to an embodiment of the presentinvention. The series of drawings presents schematic cross-sectionalviews from the side of a detector at various stages in the formation ofmetal contacts on a semiconductor substrate. In this embodiment, thesemiconductor substrate is made of cadmium zinc telluride (CdZnTe),although other semiconductor materials, such as cadmium telluride(CdTe), may also be used. Likewise, the metal used for the metallizationlayer and the metal contacts is gold, although other metals, alloys orconductive materials, including platinum and indium, may also be used.

In the following description, various method steps are described withreference to a corresponding one of the series of FIGS. 1A-1I. Forexample, “Step A” corresponds to FIG. 1A, “Step B” corresponds to FIG.1B, and so on.

Step A

A detector substrate 1 comprising, for example, CdZnTe, includes anupper face and a lower face. The lower face is uniformly metallized withgold 2.

Step B

Photoresistive material, or photoresist, is spun on the bare upper faceof detector substrate 1 to form a first photoresist layer 3. Thephotoresist may be any of the common materials used in photolithographywhich are sensitive to certain light wavelengths for creating a patternthereon.

Step C

Openings 4 are made in the first photoresist layer 3 using anappropriate mask or other conventional technique for removingphotoresist according to a desired pattern.

Step D

Photoresist is also applied to the sides 5 of detector substrate 1 toprotect them during consequent steps.

Step E

A gold layer 6 is sputtered, evaporated or laid by electrolysisuniformly over the first photoresist layer 3 and the openings 4, suchthat the gold layer 6 covers the first photoresist layer 3 and alsocontacts the CdZnTe upper surface of detector substrate 1 at theopenings 4. The gold layer 6 and the uniformly metallized face 2 areelectrically separated by photoresist on the sides 5 (edges) of detectorsubstrate 1.

Step F

A second photoresist layer 7 is applied over the gold layer 6 and alsoover the uniformly metallized face 2.

Step G

Openings 8 are made in the second photoresist layer 7 corresponding tothe areas of gold layer 6 that need to be removed (that is, the areasnot in contact with the CdZnTe surface of detector substrate 1). Eacharea of photoresist which remains is larger than the corresponding areaof the gold layer 6 in contact with the CdZnTe surface of detectorsubstrate 1.

Step H

The unnecessary areas of gold layer 6 are etched away at openings 8 inthe second photoresist layer 7 using a gold etchant. The secondphotoresist layer 7 protects the gold contacts 9 formed from portions ofgold layer 6 which are in contact with the CdZnTe surface of detectorsubstrate 1, since the photoresist is not sensitive to the etchant. Asthe area of photoresist which remains on the upper face of detectorsubstrate 1 is larger than the corresponding area of gold layer 6 incontact with the CdZnTe surface, the etchant is prevented from reachingthe CdZnTe surface, even at the interface between the gold layer 6 andthe first photoresist layer 3.

Step I

The second photoresist layer 7 is removed, thus revealing the goldcontacts 9; and the first photoresist layer 3 is removed, thus revealingthe bare face 10 of the CdZnTe surface of detector substrate 1 betweenthe gold contacts 9. The photoresist layer on the sides 5 and the lowerface of the detector substrate 1 are also removed at this stage. Nophotoresist therefore remains on the CdZnTe surface of detectorsubstrate 1. This is desirable because photoresist is usuallyhydroscopic material that in time would absorb humidity and degrade theperformance of the detector.

As an alternative to Steps F-I above, the first photoresist layer 3 maybe removed with the unwanted portions of gold layer 6 using a techniqueknown as “liftoff.” In this case, the unwanted portions of gold layer 6may be removed without involving a second layer of photoresist andwithout using a gold etchant.

The end-result of the above-described method is a CdZnTe detector havinga lower face 2 uniformly metallized with gold and an upper facemetallized with gold contacts 9 in a desired pattern. The methodadvantageously ensures that gold etchant does not come into contact withthe CdZnTe surface at any stage. The bare face 10 between the final goldcontacts 9, or “pixel pads,” thus remains totally unharmed and is notinfluenced by the gold etchant. As a result, the surface of the CdZnTeretains very high resistivity, in excess of 1 GΩ/square between goldpixel pads 9, and very low surface leakage current. As mentioned above,as high as possible resistivity between gold pixel pads 9 is desired inorder to allow long integration, standby or readout times of the signalcreated from impinging X-rays and gamma-rays without deterioration ofthe image contrast resolution. With the above-described method, theinter-pixel resistivity can be tens, hundreds or even few thousands ofGΩ/square without compromising pixel resolution. Indeed 300 GΩ/squarehas been measured and values in excess of a TΩ/square arc achievable.Moreover, using the above-described method, gold pixel pads 9 as smallas 10 μm across with 5 μm spacing in between (i.e., 15 μm positionsensitivity) can be readily obtained, while still retaining very highinter-pixel resistivity.

For at least some applications, it is desirable to add a passivationlayer between each of the gold pixel pads to ensure a stable performanceover time by avoiding oxidation of the surface not covered by gold.Passivation also enhances inter-pixel resistivity. One problem, however,is the compatibility of the passivation layer with respect to CdZnTe.The inventors have found that aluminum nitride is an appropriatepassivation material for CdZnTe.

FIGS. 2A-2K illustrate in progressive fashion a method for forming metalcontacts on a semiconductor substrate at positions for definingradiation detector cells with a layer of passivation material betweenthe metal contacts, according to another embodiment of the presentinvention. The same materials may be used as those discussed withreference to the embodiment of FIGS. 1A-1I. In addition, the passivationmaterial may comprise aluminum nitride, although other materialscompatible with the substrate material could also be used. As in thediscussion with reference to FIGS. 1A-1I, various method steps aredescribed with reference to a corresponding one of the series of FIGS.2A-2K.

Step A

A detector substrate 1 comprising, for example, CdZnTe, includes anupper face and a lower face. The lower face is uniformly metallized withgold 2.

Step B

A passivation layer 11 is formed by sputtering aluminum nitride on thebare CdZnTe upper face of detector substrate 1.

Step C

Photoresistive material, or photoresist, is spun on passivation layer11, forming a first photoresist layer 12.

Step D

Openings 13 are made in the first photoresist layer 12 using anappropriate mask or other conventional technique for removingphotoresist according to a desired pattern.

Step E

Photoresist is also applied to the sides 14 of detector substrate 1 toprotect them during consequent steps.

Step F

Openings 15 are made through the passivation layer 11 using an aluminumnitride etchant to expose the CdZnTe surface of the detector substrate1.

Step G

A gold layer 16 is sputtered, evaporated or laid by electrolysisuniformly over the first photoresist layer 12 and the openings 15, suchthat the gold layer 16 covers the first photoresist layer 12 and alsocontacts the CdZnTe upper surface of detector substrate 1 at theopenings 15. The gold layer 16 and the uniformly metallized lower face 2are electrically separated by the photoresist on the sides 14 (edges) ofdetector substrate 1.

Step H

A second photoresist layer 17 is applied over the gold layer 16 and theuniformly metallized lower face 2.

Step I

Openings 18 are made in the second photoresist layer 17 corresponding tothe areas of gold layer 16 that need to be removed (that is, the areasnot in contact with the CdZnTe surface of detector substrate 1). Eacharea of photoresist which remains is larger than the corresponding areaof the gold layer 16 in contact with the CdZnTe surface of detectorsubstrate 1.

Step J

The unnecessary areas 19 of gold layer 16 are etched away at openings 18in the second photoresist layer 17 using a gold etchant. The secondphotoresist layer 17 protects the gold contacts 21 formed from portionsof gold layer 16 which are in contact with the CdZnTe surface ofdetector substrate 1, since the photoresist is not sensitive to theetchant. As the area of photoresist which remains on the upper face ofdetector substrate 1 is larger than the corresponding area of gold layer16 in contact with the CdZnTe surface, the etchant is prevented fromreaching the CdZnTe surface, even at the interface between the goldlayer 16 and the first photoresist layer 12.

Step K

The second photoresist layer 17 is removed, thus revealing the goldcontacts 21; and the first photoresist layer 12 is removed, thusrevealing the passivation layer 11 in the regions 20 between the goldcontacts 21. The photoresist layer on the sides 14 and the lower face ofthe detector substrate 1 are also removed at this stage. No photoresisttherefore remains on the CdZnTe surface of detector substrate 1. This isdesirable because photoresist is usually hydroscopic material that intime would absorb humidity and degrade the performance of the detector.

The method of the foregoing embodiment advantageously ensures thatneither gold etchant nor aluminum nitride etchant comes into contactwith the regions 20 between the gold contacts 21 or the edges and sidesof the CdZnTe surface of detector substrate 1. Consequently, during theabove procedure the surface of the detector substrate 1 at the regions20 between the gold contacts 21 remains unharmed, retaining very highresistivity on the order of GΩ/square, tens, hundreds, or even thousandsof GΩ/square. The aluminum nitride passivation layer 11 covers theregions 20 between the gold contacts 21, protecting the correspondingregions from oxidation (providing stability over time) and enhancinginter-contact resistivity.

Numerous variations of the embodiment described above with reference toFIGS. 2A-2K are possible without departing from the spirit and scope ofthe present invention. For example, the first photoresist layer 12 maybe removed prior to gold sputtering (after openings 15 have been made).This alternative method is illustrated progressively in FIGS. 3A-3L.Applying the same naming convention used above, Steps A-F (respectivelyillustrated in FIGS. 3A-3F) correspond to Steps A-F illustrated in FIGS.2A-2F. The remaining method steps are described below.

Step G

Photoresist is removed from the upper face to expose the passivationlayer 11 at regions 22.

Step H

This step corresponds generally to Step G of the previous embodiment(illustrated in FIG. 2G), except that here the gold layer 24 is appliedover the passivation layer 11 at regions 22 and over the bare surface ofdetector substrate 1 at openings 23.

Steps I-L

These steps correspond generally to Steps H-K of the previous embodiment(illustrated in FIGS. 2H-2K), except for the absence of the firstphotoresist layer 12.

A result of the method illustrated in FIGS. 3A-3L is that the resultantpixel pads are flatter (i.e., they have a lower profile) than with themethod of FIGS. 2A-2K, as can be seen by comparing FIG. 2K and FIG. 3L.

FIGS. 4, 5 and 6 illustrate a number of possible pixel contact patternsdisposed on the upper surface of a detector substrate. In FIG. 4, forexample, an array of square pixel contact pads is arranged on a detectorsubstrate. By contrast, FIG. 5 illustrates an array of circular pixelpads. The use of circular, rather than square, pixel pads increases thesurface resistance between pads by increasing the amount of resistivematerial between adjacent pads. Similarly, FIG. 6 illustrates an arrayof offset (honeycombed) pixel pads. Once again, such an arrangementincreases the resistance between pads by increasing the surface amountof resistive material between adjacent pads. Persons skilled in the artwill recognize that, rather than providing an array of contacts fordefining an array of pixel detector cells, embodiments of the presentinvention may be used to create other contact configurations, such ascontact strips for defining strip-shaped detector cells.

In the foregoing embodiments, the metal contacts formed by the describedmethods are comprised of gold. Gold is an advantageous material for thispurpose because it can be readily etched to define desired contactstructures and give good contact (better than aluminum, for example) tothe CdZnTe. Nevertheless, the foregoing embodiments could also beapplied for any kind of metal contacts (e.g., platinum) for which anappropriate etchant is available.

As mentioned above, the longitudinal dimensions (width) of the top ofthe gold contacts 9 (FIG. 11), 21 (FIG. 2K) or 31 (FIG. 3L) is largerthan that at the gold-substrate interface. This arises from the relativesizes of the openings to the substrate surface and the photoresist leftover the portions for forming the contacts to ensure that, whenredundant gold is etched away, the etchant will not seep through to theinterface between the first photoresist layer (or the passivation layer)and gold in the openings.

According to yet another embodiment of the present invention, aradiation imaging device (700) may be constructed by connecting aradiation detector (701) (produced according to one of the methodsdescribed above) to a readout chip (702) having circuits foraccumulating charge from successive radiation hits, with individualcontacts (e.g., pixel pads) for respective detector cells being“flip-chip” joined (e.g., by bump bonding using balls of indium orconductive polymer material, gluing using one-way conductive materials,or other conductive adhesive layer techniques) to respective circuitsfor accumulating charge.

Thus, the present invention provides a method for obtaining a radiationdetector (e.g., based on a CdZnTe substrate) with one side metallizedaccording to a desired pattern with maximum possible electricalresistivity separation between the metal contacts. High resistivitybetween metal contacts is desirable to improve contrast resolution andeliminate signal leakage between adjacent metal contacts on thesubstrate surface. This is particularly advantageous when long chargeaccumulation times and long standby/readout times are employed by thereadout chip. Such accumulation and standby/readout times could, forexample, be in excess of I msec in implementations of imaging devicesusing a radiation detector manufactured according to an embodiment ofthe present invention. Such imaging devices find application, forexample, in X-ray, gamma-ray and beta-ray imaging as described in theApplicant's International patent application PCT EP95/02056, thedisclosure of which is expressly incorporated herein by reference.

Although particular embodiments of the present invention have beendescribed above by way of example, persons skilled in the art willappreciate that additions, modifications and alternatives thereto may beenvisaged within the spirit and scope of the invention.

What is claimed is:
 1. A radiation detector comprising a semiconductorsubstrate for detecting radiation with a plurality of metal contacts forrespective radiation detector cells on a first surface thereof and witha layer of conductive material on a second surface of said substrateopposite to said first surface, wherein said substrate is formed fromcadmium zinc telluride semiconductor material for detecting x-rays,gamma-rays or beta-rays, said cell contacts and said layer of conductivematerial are on said first and second surfaces, respectively, of saidsemiconductor material and aluminum nitride passivation material extendsbetween individual contacts on said first surface of said substrate. 2.A radiation detector comprising a semiconductor substrate for detectingradiation with a plurality of metal contacts for respective radiationdetector cells on a first surface thereof and with a layer of conductivematerial on a second surface of said substrate opposite to said firstsurface, wherein said substrate is formed from cadmium telluridesemiconductor material for detecting x-rays, gamma-rays or beta-rays,said cell contacts and said layer of conductive material are on saidfirst and second surfaces, respectively, of said semiconductor materialand aluminum nitride passivation material extends between individualcontacts on said first surface of said substrate.
 3. A radiationdetector according to claim 1 or 2, wherein said metal contacts definean array of pixel cells.
 4. A radiation detector according to claim 3,wherein said contacts are substantially circular and are arranged in aplurality of rows, with alternate rows being offset from adjacent rows.5. A radiation detector according to claim 4, wherein said metalcontacts define a plurality of strips arranged parallel to each other.6. A radiation detector according to claim 1 or 2, wherein theresistivity between metal contacts is in excess of 10 GΩ/square.
 7. Aradiation detector according to claim 1 or 2, wherein at least one ofsaid plurality of metal contacts comprises a rim upstanding from saidfirst substrate surface.
 8. A radiation imaging device comprising aradiation detector in accordance with claims 1 or 2 and a readout chiphaving a circuit for accumulating charge from successive radiation hits,individual contacts for respective detector cells being connected by aflip-chip technique to respective circuits for accumulating charge. 9.The radiation imaging device according to claim 8, wherein the radiationis one of the group consisting of X-ray, gamma-ray and beta-rayradiation.
 10. A radiation detector comprising a semiconductor substratefor detecting radiation with a plurality of metal contacts forrespective radiation detector cells on a first surface thereof and witha layer of conductive material on a second surface of said substrateopposite to said first surface, wherein said substrate is formed fromcadmium zinc telluride semiconductor material for detecting x-rays,gamma-rays or beta-rays, said cell contacts and said layer of conductivematerial are on said first and second surfaces, respectively, of saidsemiconductor material and passivation material extends betweenindividual contacts on said first surface of said substrate, and saidmetal contacts are of the order of 10 μm across with a spacing of theorder of 5 μm.
 11. A radiation detector comprising a semiconductorsubstrate for detecting radiation with a plurality of metal contacts forrespective radiation detector cells on a first surface thereof and witha layer of conductive material on a second surface of said substrateopposite to said first surface, wherein said substrate is formed fromcadmium telluride semiconductor material for detecting x-rays,gamma-rays or beta-rays, said cell contacts and said layer of conductivematerial are on said first and second surfaces, respectively, of saidsemiconductor material and passivation material extends betweenindividual contacts on said first surface of said substrate, and saidmetal contacts are of the order of 10 μm across with a spacing of theorder of 5 μm.
 12. A radiation detector comprising a semiconductorsubstrate for detecting radiation with a plurality of metal contacts forrespective radiation detector cells on a first surface thereof and witha layer of conductive material on a second surface of said substrateopposite to said first surface, wherein said substrate is formed fromcadmium zinc telluride semiconductor material for detecting x-rays,gamma-rays or beta-rays, said cell contacts and said layer of conductivematerial are on said first and second surfaces, respectively, of saidsemiconductor material and passivation material extends betweenindividual contacts on said first surface of said substrate, and theresistivity between metal contacts is in excess of 1 GΩ/square.
 13. Aradiation detector comprising a semiconductor substrate for detectingradiation with a plurality of metal contacts for respective radiationdetector cells on a first surface thereof and with a layer of conductivematerial on a second surface of said substrate opposite to said firstsurface, wherein said substrate is formed from cadmium telluridesemiconductor material for detecting x-rays, gamma-rays or beta-rays,said cell contacts and said layer of conductive material are on saidfirst and second surfaces, respectively, of said semiconductor materialand passivation material extends between individual contacts on saidfirst surface of said substrate, and the resistivity between metalcontacts is in excess of 1 GΩ/square.
 14. A radiation detector accordingto claim 12 or 13, wherein the resistivity between metal contacts is inexcess of 100 GΩ/square.
 15. A radiation detector according to claim 12or 13, wherein the resistivity between metal contacts is in excess of1000 GΩ/square.